# Thermodynamics: latent heat

1. Mar 12, 2013

### cdot

I need some clarifications on the precise definitions and scope of latent heat. My textbook defines latent heat as the amount of energy absorbed or releases during a phase change process. So if I have a pot of liquid water and I vaporize it completely, then the latent heat is simply the energy I had to put into the system to completely vaporize the water? What if I only vaporize half of the liquid water? Would the latent heat be the energy I had to put into the system in order for that to happen or does latent heat not apply to this situation because not all of the water is vaporized?

2. Mar 12, 2013

### Simon Bridge

Latent Heat is the amount of energy needed to change the state of a substance without changing it's temperature. That's the concept. The quantity is usually defined to be the amount of heat needed to change the state of a standard reference mass of the substance since it clearly takes more heat to change the state of more substance.

eg. - the latent heat of vaporizaton for water is 2257kJ/kg ... i.e. you need 2257kJ to vaporize 1kg of water at 100degC (at std pressure). If you vaporized half a kg, that would need 1123.5kJ or half as much.

If you supplied 2.257kW of heat to the water, then it would take 1000s to vaporize 1kg of water.